1. Field of the Invention
This invention relates to a thermostat with a load relay cycling feature, and to a method of controlling an HVAC system utilizing load relay cycling.
2. Description of the Prior Art
Thermostats used for controlling HVAC systems typically control pulsed latching relays in the HVAC systems to operate heating loads and cooling loads. Thermostats in systems using such relays cannot confirm whether the heating unit or the cooling unit, as the case may be, has actually responded when an attempt is made to activate the unit. Without the ability to confirm activation of the load, prior art thermostats are unable either to make repeated attempts to activate the heating or cooling unit, or to signal an actual failure of control. Failure of control will result in heating or cooling not actually being performed when needed, or heating or cooling continuing when it is not wanted. Aside from being wasteful in the latter case, failure of control may result in the environmental temperature becoming uncomfortable.
Also, in some heating systems, even though there is no fault in the controlling thermostat, a lockout condition can occur because an ignition controller (for example) may have detected a momentary or transient problem. Such momentary problems may include, in a gas heating system, a drop in the available gas pressure such that the ignition system cannot ignite the furnace. Other conditions may also be detected that could cause a lockout in gas or other types of systems, including heat pump systems. Various temperature control units (typically, gas heating systems and heat pumps) detect potentially troublesome or, perhaps, hazardous conditions and enter into a lockout mode to prevent the unit from being operated by the thermostat.
Although lockout conditions are generally temporary, not all furnaces or heating systems are provided with means for automatically restoring thermostat control when the condition causing the lockout has passed. For example, in the case of a momentary gas pressure drop, a conventional thermostat may remain "locked out" so as not to be able to reactivate the furnace, even though heating may be required. Moreover, a conventional thermostat also has no means for detecting, signaling, and/or correcting this condition.